This invention relates to a voltage detector including an optical modulator for detecting voltages of an object being measured with a time resolution of the order of picoseconds.
A voltage detector using an optical modulator to detect voltages of the object being measured has been known in the art. FIG. 1 and 2 are block diagrams for showing the arrangements of the conventional voltage detectors which have been disclosed in U.S. Pat. No. 4,446,425 and European Patent Unexamined Application No. 0,197,196 published by European Patent Office on Oct. 5, 1986, respectively.
In the voltage detector as shown in FIG. 1, a pulse light source 50 outputs a short light pulse of the order of 120 femtoseconds repeatedly. The short light pulse thus outputted is split into two light pulses by separating means such as a splitter (not shown). One of the light pulses is applied through a chopper 51 and a variable delay unit 52 to an object 53 being measured such as a photo-electric switch, while the other light pulse is applied to an optical modulator 40. The optical modulator 40 comprises a polarizer 55, a Pockels cell 54, a phase compensator 56, and an analyzer 57. Utilizing the phenomenon that the short light pulse applied to the modulator is modulated with a voltage provided by the object 53 being measured, the optical modulator 40 can detect a voltage waveform of the object 53 as optical intensities.
This mechanism will be described in more detail. A voltage to be detected, which is outputted by the object 53 in synchronization with the short light pulse, is applied to the Pockels cell 54 in the optical modulator 40, while a specified polarization component extracted from the short light pulse by the polarizer 55 is applied to the Pockels cell 54. The Pockels cell 54 employs an electro-optic material such as LiNbO.sub.3 or LiTaO.sub.3 whose refractive index is changed by applying a voltage thereto. Owing to the above nature of the electro-optic material, the short light pulse applied to the Pockels cell 54 is changed in polarization according to the voltage applied from the object 53 to the Pockels cell. That is, the short light pulse is modulated with the voltage of the object to emit from the Pockels cell. The emergent light beam from the Pockels cell is applied through the phase compensator 56 to the analyzer 57. The analyzer 57 extracts two orthogonal polarization components of the emergent light beam from the phase compensator 56, and applies the two orthogonal polarization components as modulated optical intensity signals to photodetectors 58 and 59, respectively. The photodetector 58 and 59 detect the optical intensities of the polarization components, respectively. The output signals of the photodetectors 58 and 59 are applied to a differential amplifier 60, where the difference between the output signals is amplified. The output of the differential amplifier 60 is applied through a lock-in amplifier 61 and a signal averager 62 to display 63, so that the result of detection is displayed on the display 63.
The variable delay unit 52 is used to gradually delay the time at which the object 53 generates a voltage, thereby to determine sampling points for the voltage waveform. The lock-in amplifier 61 is used to amplify only the specified frequency component of the output signals from differential amplifier 60, which is determined by the frequency of the chopper 51, so that noise components are reduced The signal averager 62 operates to average the output signals of the lock-in amplifier 61.
When a voltage V is applied to the Pockels cell 54 of the optical modulator 40 in the voltage detector thus constructed, the optical intensity I of the emergent light beam which is applied to the photodetector 59 shows the V-I characteristic as shown in FIG. 3 (a). When no output voltage of the object 53 is applied to the Pockels cell 54 in the optical modulator 40, the optical intensity I of the emergent light beam applied to the photodetector 59 is changed in correspondence with the set value of the phase compensator 56. Therefore, if the set value of the phase compensator 56 is so selected that the optical intensity of the emergent light beam applied to the photodetector 59 is 50% of the maximum optical intensity I.sub.0, as is apparent from the V-I characteristic as shown in FIG. 3 (a), the optical modulator is operated in such a manner that a voltage of an operating point V.pi./2 is seemingly applied to the Pockels cell where V.pi. is the voltage required for obtaining the maximum optical intensity I.sub.o and is called a "half-wave voltage". In this case, the operating point is set at point A. When with the phase compensator 56 thus set, a modulation voltage .DELTA.V from the object 53 as shown in FIG. 3 (b) is applied to the optical modulator 40, the optical intensity I of the emergent light beam incident from the analyzer 57 to the photodetector 59 is shown in FIG. 3 (c). As is apparent from FIG. 3 (a) to FIG. 3 (c), at the operating point A the optical intensity I of the emergent light beam from the analyzer 57 changes most remarkably and substantially in proportion to the applied voltage, so that the maximum AC component I.sub.AC can be obtained. On the other hand, at the operating point A, the optical intensity I includes a DC component I.sub.DC. However, as the difference between the two output signals having opposite phases, which are provided by the photodetectors 58 and 59, is amplified by the differential amplifier 60, the DC component I.sub.DC is removed and only the AC component I.sub.AC can be accurately detected as a voltage detection result. The output of the differential amplifier 60 is applied to the lock-in amplifier 61, where only the frequency component determined by the frequency of the chopper, for example 1KH.sub.Z is amplified, with the result that the noise components are reduced.
In the voltage detector as shown in FIG. 2, as a CW light beam from a CW light source 70 is applied through an optical modulator 40 to a streak camera 71, the optical intensity of an emergent light beam from a polarizer 57, which changes with the output voltage of the object 53, is observed with the streak camera The output of the streak camera 71 is applied through a lock-in amplifier 61 and a signal averager 62 to a display unit 63, to thereby detect the voltage. The generation of a voltage by the object 53 and the operation of the lock-in amplifier 61 are in synchronization with the output pulse of a pulse generator 72. A phase shifter 73 causes a sweep voltage applied to a deflector (not shown) in the streak camera 71 to be gradually shifted in phase from the output pulse of the pulse generator 72.
As the voltage detector thus constructed employs the streak camera 71 as a photodetector, the operating point is necessarily set at B in FIG. 3 (a). That is, it is impossible to provide a streak camera with a large dynamic range, and if the DC component I.sub.DC of the optical intensity I is large, then the AC component I.sub.AC as a signal to be detected cannot be observed. Therefore, the set value of the phase compensator 56 is so determined that the optical intensity of the light beam applied to the streak camera 71 is minimum when the voltage V applied to the optical modulator 40 is zero (0) volt, and therefore the operating point is set at B.
At the operating point B, the DC component I.sub.DC can be made extremely small, and therefore the modulation degree MOD that is a ratio of I.sub.AC to I.sub.DC can be made maximum. Thus, it is also possible to observe the AC component I.sub.AC with the streak camera 71 having a small dynamic range. The AC component I.sub.AC is reduced at the operating point B, more than at the operating point A. However, it is increased by the function of multiplication of the streak camera 71 so that it can be observed.
In the voltage detector as shown in FIG. 1 the maximum AC component I.sub.AC can be obtained at the operating point A, however, the optical intensity I also includes the DC component I.sub.DC. Therefore, the conventional detector is still disadvantageous in that it cannot use a streak camera which functions as a general high-speed photodetector having a small dynamic range, and must use a pulse light source expensive in price and difficult to operate.
On the other hand, in the voltage detector as shown in FIG. 2, the DC component I.sub.DC is greatly reduced at the operating point B, and therefore the CW light source 70 and the streak camera 71 can be used in combination. However, the detector is also disadvantageous in that, as the streak camera having small dynamic range is used the operating point is fixed at B, that is it cannot be freely changed, and the detector is short of flexibility.
Further, in the voltage detectors as shown in FIGS. 1 and 2, it is possible to obtain the maximum AC component I.sub.AC or the maximum modulation degree MOD, however, there is no maximization of the S/N ratio.
Still further, in the voltage detector as shown in FIG. 1, the noise components are removed from the output of the differential amplifier 60 by means of the lock-in amplifier 61. However, since the differential amplifier 60 applies the difference between the output signals of the photodetectors 58 and 59 having a low response speed to the lock-in amplifier 61, the signal to be detected is greatly deformed when applied to the lock-in amplifier 61, and therefore the modulated signal, that is, the AC component I.sub.AC cannot be accurately amplified by the lock-in amplifier 61.
Still further, the sampling frequency of the lock-in amplifier 61 is as low as 100 KHz owing to its time constant. Therefore, the lock-in amplifier 61 suffers from a difficulty that it is impossible to increase the sampling frequency to reduce the noise components, especially the 1/f noise component.